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Abstract

Tunability of the optical response of multilayered photonic structures has been compared with sequential (SQ) and superposition (SP) addition of refractive index profile functions. The optical response of the composite multilayered structure, formed after the SP addition of the two Bragg type refractive index profile functions has been studied as a function of percentage overlap and relative shift between the profiles. Apart from the substantial advantage in terms of the reduced physical thickness of the SP composite structures (over the SQ addition), at certain optimum values of relative shift, photonic structures with better quality factor resonant modes or a broader PBG could be designed. Similar analysis has been extended for rugate filters as well. The experimental verification of the optical response, was carried out through multilayered dielectric porous silicon structures fabricated by electrochemical anodization.

Reflectivity spectrum of a composite structure obtained by (a) SP and (b) SQ addition of two Bragg mirrors. The dotted/dashed line correspond to typical Bragg mirrors designed for λ0 = 1.362/1.5 μm. Figure (c) shows a contour plot for the reflectivity response as a function of the relative SP (Δλ/λ1) and the wavelength (λ). The first mirror is fixed at (λ2). The color scale indicates the reflectance from 0 (black) to 1 (yellow). Figure (d) shows the reflectance spectra of two SP mirrors, one is localized at 1.362 μm and the other is centered at 1.4 μm (—), 1.5 μm (- - -), 1.6 μm (···), and 1.7 μm (· – ·). Figure (e) shows the comparison of two resonant microcavity modes obtained by superposition (—) and typical half wave microcavity (· – ·) structure designed for λ0 = 1.45 μm.

(a) Contour plot of the reflectivity spectrum, as a function of wavelength and the percentage of relative shift for two Bragg mirrors under SP addition. The color scale indicates the reflectance from 0 (black) to 1 (yellow). (b) Comparison of the PBG obtained by SP addition for three different values of shift: 4.6 % or 344 nm (· – ·), 33 % or 2474 nm (—) and 100 % shift or SQ addition (- - -), revealing an increase of PBG for SP added structures. Inset shows the PBG as a function of % shift revealing an optimum value of shift for obtaining a maximum PBG

(a) Contour plot of the reflectivity spectrum (R), as a function of the wavelength (λ) and the percentage of relative overlapping (Δλ) for two rugate mirrors under superposition addition. The color scale indicates the reflectance from 0 (black) to 0.9 (yellow). (b) Contour plot of the reflectivity spectrum (R), as a function of the wavelength (λ) and the percentage of relative shift for two rugate mirrors under superposition addition. The color scale indicates the reflectance from 0 (black) to 0.7 (yellow).